Analysis for kinetics of ferrite growth due to isothermal decarburization of austenite in the binary Fe-C system

Abstract

If a binary Fe-C alloy with a single-phase microstructure of the austenitic γ phase is isothermally annealed in an appropriate decarburization atmosphere, a layer of the ferritic α phase is formed on the surface of the γ phase and gradually grows into the γ phase. The kinetics for the growth of the α layer during the decarburization was quantitatively analyzed using a diffusion model at annealing temperatures between 1011 and 1185K. In the analysis, the diffusion coefficient of C in each phase is considered independent of the chemical composition. According to the model, the square of the thickness l of the α layer is proportional to the annealing time t as described by the relationship l 2 = Kt. This relationship is called the parabolic relationship. As the initial concentration x γ0 of C in the γ phase increases from the minimum value to the maximum value for the γ single-phase region at each annealing temperature T, the parabolic coefficient K monotonically decreases from the maximum value K maxd to the minimum value K mind. As T decreases, K maxd decreases, but K mind increases. However, both K maxd and K mind vary depending on T in a complicated manner. Thus, an Arrhenius equation is not applicable even to the temperature dependence of K maxd in the whole annealing temperature range. At a constant value of x γ0, K monotonically decreases with increasing value of T. This means that the growth of the α layer takes place faster at lower annealing temperatures than at higher annealing temperatures. Such temperature dependence of the kinetics coincides well with experimental observations. © 2012 The Japan Institute of Metals.